Plagioclase Feldsjxtrs. 593 



Point on Liquidus. 



1517° 



1521° 



- 4 



1480° 



1490° 



-10 



1436° 



1450° 



-14 



1380° 



1394° 



-14' 



Case III. 







AbjAi^ 

 Al^An", 



A^An; 



Ab„An. 



Assuming a value of L Ab 10 per cent lower than above mean value 

 and a value of Z A „ 10 per cent lower than above mean value 



Point on Solidus. 

 Composition Calculated Observed Difference 



AbjAn, 1382° 1372° +10° 



A^An^ 1299° 1287° -fl2° 



Point on Liquidus. 



Ab 1 An 2 1485° 1490° -5° 



Ab 1 An i 1441° 1450° -9° 



Ab 2 An a 1386° 1394° -8° 



Case IV. 



Assuming a value of L Ah 10 per cent higher than above mean 

 value and a value of L An 10 per cent lower than above mean 

 value 



Point on Solidus. 



Composition Calculated Observed Difference 



Ab 1 An 2 1350° 1372° —22° 



Point on Liquidus. 



Ab 1 An i 1430° 1450° —20° 



The marked accord between theory and the measurements 

 here offered is, on first thought, rather surprising. We are 

 not dealing simply with dilute solutions, nor is the solid which 

 separates the pure solvent, and it is commonly only in such 

 cases that measurements of freezing point lowering are found 

 amenable to simple laws. It should be noted, however, that 

 the equations applied have the general form applicable to all 

 solutions and not merely to the limiting case of the dilute 

 solution. Moreover, the fundamental assumption in the deriva- 

 tion was that both the liquid solutions and the solid solutions 

 followed Raoult's law of vapor pressure lowering. This law 

 is well knowm to be applicable to many pairs of liquids, for all 

 concentrations, when these liquids are miscible in all propor- 

 tions and when there is neither volume change nor heat effect 

 on mixing. The feldspar liquids fulfill the first two require- 

 ments. The density of the feldspar glasses at room tempera- 



Am. Jour. Sci.— Fourth Series, Vol. XXXV, No. 210.— June, 1913. 

 42 



